This invention relates to sigma-delta analogue-to-digital conversion circuits, particularly, though not exclusively, higher order sigma-delta analogue-to-digital conversion circuits; that is conversion circuits of the order two or more.
A typical higher order sigma-delta analogue-to-digital conversion circuit has a filter comprising a plurality of individual filter circuits, such as LC filter circuits, SC filter circuits or SI filter circuits arranged in a cascaded configuration.
Analogue-to-digital conversion circuits of this kind are conditionally stable; that is, they operate satisfactorily provided the input amplitude is below a critical level. Once this level is exceeded, the sigma-delta converter starts to oscillate and loses its noise shaping features. Then even if the amplitude is decreased the coverter will continue to oscillate.
Hitherto, it has been customary to include additional analogue circuitry to monitor different internal parameters in the conversion circuit and to initiate appropriate remedial action should instability be detected. However, such additional circuitry adds unduly to the overall complexity and power consumption of the conversion circuit.